1. Field of the Invention
The invention generally relates to therapies for the treatment and prevention of certain parasitic diseases. In particular, the invention provides a novel Fasciclin Related Adhesive Protein (FRAP) as a target for therapeutic intervention in diseases caused by Plasmodium species.
2. Background of the Invention
Malaria, a blood-borne infection caused by Plasmodium parasites, is a major health issue in the tropics, with 300-500 million clinical episodes of this disease occurring each year. A licensed vaccine against malaria is not available and the parasite is developing resistance against most of the currently available antimalarials. There is an urgent need to develop new therapeutics (drugs and vaccines) against malaria, which will reduce the morbidity and mortality associated with this disease. The genome of Plasmodium falciparum has been sequenced and can be exploited to understand the molecular basis of the onset and sustenance of infection by these pathogens. Deciphering these mechanisms will unravel the complex interplay between the troika of host, pathogen and its environment, which is vital for identifying new targets for intervention.
Malaria infection starts with the introduction of Plasmodium sporozoites into the blood stream of its human host, when it is bitten by an infected mosquito. Of the four Plasmodium species that infect humans, P. falciparum is the most virulent—resulting in severe anemia and cerebral malaria, which can be fatal. Fewer than 200 sporozoites are introduced and even fewer succeed in invading liver cells, the target organ for the onset of malaria infection in a host. A successful adhesion and liver cell invasion by the sporozoite is critical for this onset and is therefore, the Achilles heel of the parasite. Once inside the liver cell, the parasite rapidly multiplies and within a few days releases thousands of parasites, which leads to the clinical pathology of this disease. Therefore, an ideal approach to control malaria is to develop a vaccine or therapeutic, which either prevents the sporozoite from infecting liver cells or destroys the parasite during liver stages of its life cycle. Such a vaccine is feasible as animals and human volunteers immunized with Plasmodium sporozoites that have been attenuated by exposure to X-Ray or gamma radiation, are protected when subsequently challenged with infectious sporozoites (Hoffman, et al. (2002) J Infect Dis,, 1155-1164; Nussenzweig et al. (1967) Protective immunity produced by the injection of x-irradiated sporozoites of Plasmodium berghei. Nature, 216, 160-162.). While this groundbreaking discovery clearly indicated that it is feasible to make a vaccine against malaria, the biggest stumbling block for malaria researchers worldwide has been to decipher the parasite antigens recognized by the host and to understand the immune mechanisms underlying this protection. Extensive immunological studies with known sporozoite antigens have concluded that this protection is not conferred due to a dominant immune response against a single antigen but is mediated by the summation of many modest humoral and cell-mediated immune responses against a large variety of antigens, many of which are currently not known (Hoffmnan, S. (1996) Malaria Vaccine Development: A multi immune response approach. ASM press, Washington, D.C.). Identification of these antigens is not only the major challenge, it is vital for the development of a successful vaccine against malaria.
Historically, antigen(s) selected as a vaccine candidate in a given pathosystem are (i) present on the surface of the pathogen, (ii) are generally involved in host-pathogen interactions and are therefore, one of the first molecules that are recognized by the host immune system (Moxon, R. and Rappuoli, R. (2002) Br Med Bull, 62, 45-58). These criteria are also valid for malaria parasite as the two major vaccine candidates viz., Circumsporozoite protein (CSP) (Cerami, C. et al. (1992) Cell, 70, 1021-1033) and Thrombospondin-related anonymous protein (TRAP) (Robson, et al. (1995) Embo J, 14, 3883-3894) are involved in the invasion of liver cells by the parasite.
Upon entering red blood cells, the Plasmodium parasite undergoes rapid multiplication giving rise to 28-32 parasites in less than 48 hours. Hemoglobin represents ˜95% of the total RBC content, and the parasite digests up to 75% of the hemoglobin, which serves as its source of amino acids. While this process of hemoglobin digestion provides the parasite with a ready source of amino acids, it also releases free heme, which in the absence of a globin moiety, is extremely toxic for the parasite (Gluzman, et al. (1994) J Clin Invest, 93, 1602-1608.). The parasite survives by effectively neutralizing toxic heme into a non-toxic and polymerized product known as hemozoin, which is chemically identical to β-hematin (Francis,et al. (1997) Annu Rev Microbiol, 51, 97-123. Most of the currently available antimalarials have been shown to be binding to free heme, which inhibits its polymerization, and the toxicity resulting from the free heme causes the death of the parasite (Slater and Cerami (1992) Nature, 355, 167-169).
Therefore, pathway(s) that lead to hemozoin formation are extremely attractive drug targets.
Unfortunately, the mechanism(s) in use by the parasite for the polymerization process is poorly understood. Two parasite proteins viz., Histidine rich protein II and III have been proposed to be responsible for this activity (Sullivan, et al. (1996) Science, 271, 219-222.), though parasites lacking either or both of the proteins make copious amounts of hemozoin without any loss of activity (Wellems, et al. (1991) Proc Natl Acad Sci USA, 88, 3382-3386). Therefore, an unknown protein(s) has been long thought to be responsible for this activity.
The prior art has thus far failed to provide satisfactory vaccines or drug therapies to combat diseases caused by parasites such as Plasmodium. There is thus an ongoing need to identify and characterize potential targets for such therapeutic intervention.